The present invention relates to an objective lens unit used for converging a light flux on an information recording surface of an optical information recording medium in an optical pickup device, the optical pickup device equipped with the objective lens unit and to an optical information recording apparatus.
Heretofore, a plastic lens has mainly be used as an objective lens that is used in an optical pickup device for recording/reproducing for optical information recording media such as CD, MO and DVD.
Since the specific gravity of the plastic lens is low, compared with a glass lens, the load on an actuator that drives the objective lens can be reduced, which makes it possible for the objective lens to follow at high speed.
Further, a plastic lens manufactured through injection molding of plastic materials in a metal mold can be manufactured on a mass-production basis at higher accuracy, by manufacturing a metal mold in a desired shape to be highly accurate. It is therefore possible to attain a high-performance and low-cost lens.
In recent years, there have been advanced studies and developments of an optical pickup device for a new high density optical disk employing a violet semiconductor laser light source having a wavelength of about 400 nm and an objective lens wherein the numerical aperture on the part of an image (NA) is raised up to the level of about 0.85. As an example, in the case of an optical disk with a numerical aperture of 0.85 and a light source wavelength of 405 nm (hereinafter referred to as “high density DVD”), information of 20-30 GB per one surface can be recorded on an optical disk having a diameter of 12 cm that is in the same size as in DVD (numerical aperture of 0.6, a light source wavelength of 650 nm and storage capacity of 4.7 GB).
In the optical pickup device for the high density DVD of this kind, it is required to record and reproduce three types of optical disks each having a different standard (recording density) such as high density DVD, DVD and CD, on a compatible basis.
Optical pickup devices having interchangeability for plural types of optical information recording media (for example, CD and DVD) have been developed, and a part of them are on the market. In Optical pickup apparatuses of this kind, there is used a laser which has a different wavelength depending on a type of the optical information recording medium. Therefore, in each of these many optical pickup apparatuses, there is formed, on an optical surface of an objective lens, a ring-shaped structure that is divided into plural ring-shaped zones which cause a prescribed optical path difference for incident light, so that a single objective lens may have interchangeability for plural types of optical information recording media (these technologies are disclosed in TOKKAI Nos. 2000-81566, 2001-195769 and 2001-51192).
In the optical information recording medium capable of recording and reproducing data, energy density (power) of a laser beam is raised, for recording data, by increasing an electric current that flows through a laser oscillator, and energy density of a laser beam is lowered, for reproducing data, by decreasing an electric current that flows through a laser oscillator.
In the optical pickup device capable of recording and reproducing data for the optical information recording medium such as high density DVD, data reading and data writing are conducted alternatively over and over again. In this case, there occurs a phenomenon that a wavelength of the laser becomes longer instantaneously for raising energy density of a laser beam emitted from the laser oscillator quickly, when the state of reading is switched to the state of writing (“mode-hop”).
When a wavelength of the laser becomes longer, a position of a beam spot formed on an optical axis is moved by dispersion of a lens to be farther from the objective lens (“chromatic aberration”). Namely, the position of the beam spot is shifted out of an information recording surface of an optical disk, and there is the possibility of occurrence of errors when writing data on the optical disk.
In general, it is known that the shorter a wavelength of light is, the greater an amount of change of the refractive index of lens material for unit fluctuation amount of a wavelength is.
In the optical pickup device capable of recording/reproducing for high density DVD, a violet laser with a wavelength of about 400 nm is used as a light source, and an amount of fluctuation of a wavelength caused by mode-hop is several nanometers. Therefore, chromatic aberration caused by mode-hop turns out to be great, compared with an optical pickup device for CD use (light source wavelength: about 780 nm) and an optical pickup device for DVD use (light source wavelength: about 650 nm), thus, the chromatic aberration needs to be corrected.
As an objective lens wherein the chromatic aberration has been corrected, TOKKAIHEI No. 6-242373 discloses an objective lens wherein a ring-shaped structure that is divided into plural ring-shaped zones which cause a prescribed optical path difference for incident light is formed on an optical surface of the objective lens.
Further, the optical pickup device is exposed to changes of temperatures caused by fluctuations of atmospheric temperatures at the location of installation and by generation of heat resulting from operations of the device.
When the temperature of the optical pickup device rises, a wavelength of a laser beam emitted from a laser oscillator generally grows greater. Further, plastic has characteristics that the refractive index of the plastic becomes smaller when its temperature rises. In addition, a form of a plastic lens is easily changed because a coefficient of thermal expansion of plastic is greater than that of glass.
Due to these primary factors, spherical aberration (“temperature characteristic aberration”) occurs on a beam spot formed on an optical axis, under the ambient condition of higher (or lower) than the temperature assumed in the course of designing the optical pickup device. This temperature characteristic aberration needs to be corrected.
AS an objective lens wherein the temperature characteristic aberration has been corrected, TOKKAIHEI No. 11-337818 discloses an objective lens wherein a ring-shaped structure that is divided into plural ring-shaped zones which cause a prescribed optical path difference for incident light is formed on an optical surface of the objective lens.
In the case of high density DVD, a curvature on the optical surface (the optical surface closer to the optical information recording medium, in particular) of the objective lens is greater because numerical aperture NA on the part of an image of the objective lens is set to about 0.85. When a ring-shaped structure is provided on the optical surface having this great curvature for making the objective lens to have interchangeability for plural types of optical information recording media, and/or for correcting chromatic aberration, and/or for correcting temperature characteristic aberration, a pitch of the ring-shaped structure becomes extremely small (about several microns).
A metal mold used for injection molding for a plastic lens is manufactured by a method for cutting with a diamond cutting tool with a microscopic diameter called SPDT (Single Point Diamond Turning). However, in processing a metal mold with SPDT, there is a problem of declined efficiency of utilization of a laser beam caused by phase inconformity portion generated when a shape of a tip of the diamond cutting tool is transferred onto the metal mold, when forming, on the optical surface, a fine form such as a ring-shaped structure having a pitch of several microns. Though binary-optics making technology wherein photo-lithography and etching are applied repeatedly and an electron beam drafting technology represent a method of manufacturing an optical element which is suitable for forming fine shapes accurately, there have not been reported an example to manufacture an optical element having a fine form on an optical surface having great curvature such as an objective lens for high density DVD and an example to manufacture a metal mold that is used for injection molding of an optical element.
Further, even when accurate preparation of a fine form is technically possible by using a diamond cutting tool having a small tip shape within a range where declined efficiency of utilization of a laser beam caused by phase inconformity portion is not a problem, or even when manufacture of an optical element having a fine form on an optical surface having great curvature is technically possible owing to the binary-optics making technology and to the electron beam drafting technology, or even when manufacture of a metal mold used for injection molding of an optical element is technically possible, in processing of a metal mold with SPDT in the future, there still remains a problem of a decline of efficiency of utilization of a laser beam, because an influence of a shadow of a step portion of a ring-shaped structure is great, if a ring-shaped structure with a pitch of about several microns is provided on an optical surface having the great curvature.